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 Post subject: Eico 324 As An AM Transmitter? Opinions Please
PostPosted: Dec Sat 09, 2017 4:54 am 
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Joined: Jul Thu 29, 2010 5:35 am
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Location: Simonton, Texas
I noticed there seems to be a problem getting an inexpensive AM transmitter setup.

Why not use an Eico 324 which accepts external modulation and are very reasonably priced?

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 Post subject: Re: Eico 324 As An AM Transmitter? Opinions Please
PostPosted: Dec Sat 09, 2017 5:12 am 
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The modulation is limited to 30% or so and it doesn't have any matching network for the antenna. As a consequence of these two facts, the range is quite limited compared to a purpose-build part 15 AM transmitter.

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 Post subject: Re: Eico 324 As An AM Transmitter? Opinions Please
PostPosted: Dec Sat 09, 2017 5:25 am 
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Location: Simonton, Texas
Steve
Could those 2 limitations be overcome inexpensively?

A circuit mod for the modulation short coming and a variable LC circuit for antenna matching?

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 Post subject: Re: Eico 324 As An AM Transmitter? Opinions Please
PostPosted: Dec Sat 09, 2017 12:05 pm 
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Joined: Dec Sun 02, 2007 3:20 pm
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Location: Fargo, North Dakota
I have been able to get reasonably good performance using an URM25D signal generator as a transmitter. This was a few years ago, but I do not recall using any matching network for the antenna - just a piece of random wire. I did use a transformer to match the input impedance of the generators audio input with the output impedance of the audio source.

Here are some links to what you are trying to achieve...

viewtopic.php?f=12&t=261948&hilit=urm25&start=0

http://boatanchorpix.x10host.com/SigGenFun.htm


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 Post subject: Re: Eico 324 As An AM Transmitter? Opinions Please
PostPosted: Dec Sat 09, 2017 1:20 pm 
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Joined: Jun Fri 19, 2009 6:34 pm
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Location: Long Island
People have been doing that with small signal generators for ages. But it seldom works out well for a variety of reasons. First and foremost, the "AM sound" we are used to from commercial broadcast stations is the result of a lot of audio compression and processing, and modulation right up to 100%. Without those details, a signal generator as an AM transmitter ends up sounding very flat and lifeless. Kit/hobbyist signal generators do not have the audio compression or processing, frequency range, or depth of modulation. More elaborate signal generators can provide improved frequency response and lower distortion, and higher modulation levels, but they still lack the audio input processing. Add to that the fact that kit/hobby grade generators output microwatts or perhaps only a few milliwatts of power at most, so even if you have an external matching network for the antenna, the signals are not going to go very far--maybe not even all the way across the room you're in.

There are small AM transmitters designed for home use which overcome these limitations, the SSTran* being a popular one. They have the audio processing and modulation down to the point where it does sound like commercial AM. There are a number of other options such as a "Talking House" transmitter, and even some homebrew projects that can be built--check online.

*Cited only as an example. Sadly, SSTran recently announced that they are not accepting new orders and may not even be able to fill their backlog.

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 Post subject: Re: Eico 324 As An AM Transmitter? Opinions Please
PostPosted: Dec Sat 09, 2017 2:57 pm 
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Location: Detroit, MI USA
The signal generator option works better if it's RF output is directly connected to the antenna input of the radio being used on sets that don't have a loop. You can use a second loop connected to the generator and placed very close to a built-in antenna on sets which have one. As noted, far from perfect but usable, and the range issues are eliminated. You aren't going to be able to use this to transmit more than a few feet at best.

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 Post subject: Re: Eico 324 As An AM Transmitter? Opinions Please
PostPosted: Dec Sat 09, 2017 4:41 pm 
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Joined: Feb Sun 02, 2014 9:13 pm
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Location: Roanoke, VA
Koby wrote:
Could those 2 limitations be overcome inexpensively?

No. A service-grade signal generator uses a directly modulated oscillator, and that modulation scheme is limited to about 30% modulation. Beyond that point envelope distortion generally increases causing significant audio distortion in the received signal. Incidental FM also increases which is also objectionable. Even below 30% modulation a 324 may sound poor with external audio modulation.

Trying to modify a 324 to serve as a transmitter is, IMHO, a waste of effort. Tubed AM broadcast transmitters and tubed lab-grade signal generators buffer the oscillator and modulate the buffered RF, eliminating the artifacts of the directly modulated oscillator. To do that with the 324 would requites adding an oscillator buffer stage and a higher-power modulator stage.

If you want to use a signal generator as a transmitter you should buy a lab-grade generator. My GR 805-C standard signal generator, after restoration a few years ago, has virtually the same fidelity as the better AM broadcast transmitters of its vintage. (See http://plymouthcolony.net/starcity/radi ... -805c.html for info about the restoration)

Dale H. Cook, Radio Contract Engineer, Roanoke/Lynchburg, VA
http://plymouthcolony.net/starcityeng/index.html


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 Post subject: Re: Eico 324 As An AM Transmitter? Opinions Please
PostPosted: Dec Sat 09, 2017 4:49 pm 
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Chris108 wrote:
... the "AM sound" we are used to from commercial broadcast stations is the result of a lot of audio compression and processing, and modulation right up to 100%. Without those details, a signal generator as an AM transmitter ends up sounding very flat and lifeless.

It is possible to add used commercial processing equipment to a lab-grade signal generator to get a classic "AM sound" but it is not simple, and not something I would recommend for someone who has no experience with commercial broadcast processing. It is easy to make processing sound really bad and much more difficult to make it sound really good, as the latter requires an understanding of the building block stages used in processing (AGC, compression, limiting and clipping) and how they are used together.

I do so successfully, but I am working with perhaps the finest lab-grade generator of its time (GR 805-C) and a commercial processor (Cutting Edge Unity 2000), and I have nearly 50 years of experience with broadcast processing.

Dale H. Cook, Radio Contract Engineer, Roanoke/Lynchburg, VA
http://plymouthcolony.net/starcityeng/index.html


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 Post subject: Re: Eico 324 As An AM Transmitter? Opinions Please
PostPosted: Dec Sat 09, 2017 11:52 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Black Hills, SD 57745
Signal Generators of the kit-built or service-grade quality might not be the best, but if you already own one or could buy one and use it to service radios as well, why not try it? Besides, some of them have a really cool retro look. :) If you're shopping, be prepared to recap and repair most any vintage unit you may find.

Of the two generators I own, the post-WWII cheapy built from surplus was weak, but the Superior TV-50 had a stronger output and I thought very pleasing audio. I got around the weak signal by running the long "antenna" lead very near the radio I wanted to listen to. Still, only useful in the same room and it didn't prove it's worth to keep using it as a broadcaster, once I got an SStrans. But if most of the radios you might use are in the same area, run the output wire behind all and it might suit you just fine.

No doubt better DIY alternatives in the homebrew sub-forum. Till somebody sees the SStrans vacuum and decides to take a stab at filling it.
-Ed


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